Statistical Risk of a Severe Vaccine Reaction

All of the below is from “

The Vaccine Book” by Dr. Robert Sears (October 2007; pages 164 – 168), which provides much more detail. Some sections are copied verbatim, paraphrased or summarized.

Vaccine critics propose that the risk of a severe vaccine reaction may equal or exceed the risk of having a severe case of a disease. At best, this fear is simply theoretical. I have never found a complete analysis of this issue, one that compares all vaccine-preventable diseases with all severe vaccine reactions. However, I do think that there are enough data available to provide a basic understanding of the likelihood that your child may suffer a severe vaccine reaction. I will also review for you the statistical possibility that your child may suffer a severe case of a preventable disease…

… From data collected by the CDC between 1991 and 2001, it found the following:

1.9 billion doses of vaccines were given in the United States.

18,000 severe side effects were reported

These effects resulted in a prolonged hospital stay, a severe life-threatening illness, a permanent disability, or death.

Using these numbers, this suggests a 1 in 100,000 chance for each dose the child gets.

During the 1990s, when these data were collected, kids got about 18 shots during their childhood, which suggests there was about a 1 in 5500 chance that any one child might suffer a bad reaction during the entire vaccine schedule in the 1990s.

… Today, a child generally gets 39 total doses of vaccines on the current twelve-year schedule, which suggests a 1 in 2500 chance he will have a severe reaction over the entire twelve-year period.

… There is, unfortunately, another way to look at these numbers… Since kids get an average of 3 shots at a time (39 doses spread out over about 13 doctor visits), we can evaluate likelihood of a severe vaccine reaction per event. This suggests a 1 in 35,000 chance of a serious reaction for each round of shots — a higher likelihood than the 1 in 100,000 calculated for each individual shot. Doing the math this way suggests that a child takes a 1 in 35,000 risk each time he gets a round of shots, and he takes this risk 13 times. Then divide 35,000 by 13 (the chance of a reaction over all 13 rounds), and you get about a 1 in 2700 chance of a severe reaction throughout childhood.

In summary, the risk of a serious vaccine reaction can be calculated as:

1 in 100,000 chance for each separate vaccine

1 in 35,000 chance for each round of vaccines

1 in 2600 chance for the entire twelve-year vaccine schedule

Keep in mind that these data are based on only the known reactions that occur close enough to a vaccine for a report to be made. We know that not all vaccine reactions are reported, but we don’t know exactly how many go unreported. Is the real number of reactions double what I’ve put here? Is it triple or more? If so, that would make the chance of a reaction occurring over the entire schedule about 1 in 1300 to 1 in 900. But we don’t actually know those numbers, so instead I must work with the numbers that we do know.

The data also don’t take into account unknown factors, such as whether vaccines cause chronic, underlying problems. Some vaccines critics worry that conditions like attention-deficit / hyperactivity disorder, autism, learning disorders, neurologic problems, and autoimmune diseases, issues that seem to plague our nation’s children much more now than they did twenty years ago, may be triggered in part by the increased number of vaccines given today. If such problems indeed are caused by vaccines, then the risk of vaccines would far exceed the risks of the diseases themselves. But there’s really no way to determine this risk accurately, because we just don’t know if vaccines play a role in these conditions.

One weakness in this analysis is that it is based on data from 1991 through 2001. Now we give babies about twice the number of injections as we did back then. We have some new vaccines, and some older ones have been taken off the market. In order to understand vaccine risks for our kids today, we would need to look at these data using today’s vaccine schedule. So far, they aren’t available…

Besides understanding the overall risk for the complete array of vaccines, many parents naturally want to know the risk for each individual vaccine… This lack of accessible information is one of the biggest frustrations for parents who are trying to make a decision about vaccines. If you knew which vaccines had the greatest chance of a severe reaction, you might avoid them. If you knew that some vaccines virtually never caused a bad reaction, you would feel very confident in their safety and importance. I truly wish I could make this distinction available to you. All I can offer right now are these overall risks.